Deep eutectic solvents (DESs) as novel green solvents are potential options to replace inorganic acids for hydrometallurgy. Compared with inorganic acids, the physicochemical properties of DESs and their applications in recycling of spent lithium-ion batteries were summarized. The viscosity, metal solubility, toxicological properties and biodegradation of DESs depend on the hydrogen bond donor (HBD) and acceptor (HBA). The viscosity of ChCl-based DESs increased according to the HBD in the following order: alcohols < carboxylic acids < sugars < inorganic salts. The strongly coordinating HBDs increased the solubility of metal oxide via surface complexation reactions followed by ligand exchange for chloride in the bulk solvent. Interestingly, the safety and degradability of DESs reported in the literature are superior to those of inorganic acids. Both DESs and inorganic acids have excellent metal leaching efficiencies (>99%). However, the reaction kinetics of DESs are 2–3 orders of magnitude slower than those of inorganic acids. A significant advantage of DESs is that they can be regenerated and recycled multiple times after recovering metals by electrochemical deposition or precipitation. In the future, the development of efficient and selective DESs still requires a lot of attention.

Ferric arsenate complexes (FeH₂AsO₄²⁺ and FeHAsO₄⁺) formed in acidic solutions were confirmed by UV-Vis spectroscopic techniques, which transformed into gel-like material under higher pH (pH ≥ 2.38) or higher temperature (T ≥ 90 °C) conditions.

To investigate the structural properties of Cu²⁺-incorporated ZnAl-layered double hydroxide, a series of CuZnAl-layered double hydroxides with various molar ratios was prepared by the constant pH co-precipitation method. The composition, structure and morphology of the synthesized products were analyzed by using XRD, SEM, TEM, FTIR, XPS and ²⁷Al–¹H MAS-NMR techniques. The results show that all samples with or without copper present crystalline LDH with a flower-like architecture, and no other impurity was observed. The interesting observation of the study was that the incorporation of copper influences the coordination number of aluminum. Comparing with the ZnAl-LDH layers, which contained only ZnO₆ and AlO₆ octahedra, in the Cu-containing LDH layers, in addition to CuO₆, ZnO₆, and AlO₆ species, the AlO₄ species is clearly observed. A simplified explanation for the formation of AlO₄ species is associated with the Jahn–Teller effect of copper. These results suggest that there are structural defects (Al–O–Al connectivities) in the form of AlO₄ species in the Cu-containing LDHs.